THIS INVENTION relates to a material suitable for use as a light diffusing screen or rear projection screen and to a material suitable for use as a front projection screen and for analogous purposes, such as in reflective signs, etc.
It is well known that the important parameters of projection screen materials include brightness, that is the amount of light, from the image source, reaching the viewer and the angles from the perpendicular to the surface of the material over which an acceptable brightness level (usually in excess of 50% of the maximum) is maintained.
In the following, reference is made to the xe2x80x9cangle of viewxe2x80x9d. In this specification, the term xe2x80x9cangle of viewxe2x80x9d refers to the range of viewing angles of observers relative to a diffusing screen or a rear projection screen over which the apparent brightness of the screen is within 50% or more of the apparent brightness to an observer viewing the screen at right angles assuming that the light incident on the screen is incident along the normal to the screen. This figure is often referred to as xe2x80x9cfull width half maximumxe2x80x9d or xe2x80x9cfwhmxe2x80x9d. By way of example, the angle of view may be measured by illuminating a small area of the screen material by a laser beam (although some other collimated light beam may be used) directed normally (i.e. perpendicularly) to the plane of the screen and measuring the light diffused or reflected from that small area (according to whether a light diffusing screen such as a back projection screen or a reflective screen such as a front projection screen is involved), over a range of angles. Furthermore, in the following, the expression xe2x80x9cangle of view for transmissionxe2x80x9d means xe2x80x9cangle of viewxe2x80x9d defined as above for a screen of light-diffusing material illuminated by a projector or the like disposed on the opposite side of the plane of the screen from the observer, i.e. for a rear-projection screen, whilst the expression xe2x80x9cangle of view for reflectionxe2x80x9d means xe2x80x9cangle of viewxe2x80x9d defined as above for a screen of light reflective material illuminated by a projector or the like disposed on the same side of the plane of the screen as the observer, i.e. for a front projection screen.
Established methods for producing materials suitable for front projection screens and for analogous purposes include, on the one hand, the lamination of light diffusing films to specular or xe2x80x9cmirror likexe2x80x9d surfaces and, on the other hand, incorporating geometric features into such surfaces, for example corner cube arrays. Where a light diffusing material is laminated to a mirror-like surface, the angle of view for reflection is approximately double the angle of view for transmission for small to moderate angles of view. With a front projection screen produced by incorporating geometric features in a reflective surface the angle of view is determined by the geometry of such features. Where a light-diffusing material is laminated to a reflective surface, increases in angle of view to achieve appropriate reflective characteristics may be impossible or may be obtainable only to the detriment of some other important factor such as brightness (reduced if film thickness is increased), flexibility or cost. Where reflective diffusion is obtained by providing an array of geometric features, the angle of view can only be changed by changing the geometric features. The geometric features can only be changed by changing the mould, press tools or other tools used to produce these features, and this is expensive.
It is among the objects of the invention to provide an improved light diffusing material suitable for use as a back projection screen, to provide an improved material suitable for use as a front projection screen or in reflective signs or in analogous applications, and to provide improved methods of manufacturing such materials.
It is known to provide a diffusion or rear projection screen by a technique in which a layer of photopolymerisable material is exposed to a pattern of polymerising light comprising an array of spots of light (or, alternatively, an array of dark spots in a light field) to induce selective polymerisation of the material with consequent refractive index gradations throughout the material, resulting in a light dispersing or diffusing effect. Such selective photopolymerisation may be brought about, for example, by applying to one surface of a layer of photopolymerisable material supported on a substrate, an optical mask comprising an array of light-transmitting areas in an opaque surround (or alternatively, a mask comprising an array of opaque areas in a transparent surround). The technique may be substantially that disclosed in European Patent No. 0294122 in which the mask apertures are substantially circular, resulting in the production of an array of graded refractive index lenses in the photopolymerised layer. Alternatively, the mask apertures may be oval or ellipsoidal in shape, with their major axes extending parallel with one another, thereby producing an array of astigmatic graded refractive index lenses, as a result of which the photopolymer layer has asymmetric characteristics as regards angle of view. Techniques disclosed in co-pending International Patent Application No. PCT/GB94/01281 and in corresponding U.S. Pat. No. 5,695,895 allow the production of a mask in which the array of apertures in the mask conforms to a random xe2x80x9cspecklexe2x80x9d pattern, in which individual apertures are irregularly and randomly shaped.
In the techniques described above, the polymerising light is incident on the mask and underlying photopolymer layer substantially normally. However, the applicants have discovered that useful properties can be imparted to the resulting screen if, during exposure the light or other polymerising radiation is directed into the photopolymerisable material at an angle other than substantially normally, or if multiple sources of light or other polymerising radiation are used to create multiple beams, if at least some of these beams are inclined to the plane of the photopolymer layer. The applicants have also found that by adopting such expedients whilst at the same time ensuring that the exposure arrangement is symmetrical about a preferred axis, for example an axis normal to the layer of polymerisable material, symmetry of the resulting diffusion characteristics about such axis can be obtained. Such symmetry can also be obtained by rotation of the assembly including the layer of polymerisable material relative to the exposure means about such axis during exposure. According to one aspect of the invention there is provided a method of making a light-diffusing screen comprising providing a sheet of a light-transmitting radiation polymerisable (as herein defined) material of a type in which, after polymerisation, the refractive index varies with degree of polymerisation, exposing said material to a predetermined pattern of polymerising radiation comprising an array of spots of high intensity radiation in a background of lower or zero intensity radiation, or comprising an array of spots of lower or zero intensity radiation in a background of high intensity radiation, at least some of said radiation being directed along an axis which is inclined with respect to the plane of said sheet of material.
According to another aspect of the invention there is provided a light-diffusing sheet material having at least some graded refractive index features having axes or preferred orientations aligned along a direction having a component parallel with the plane of the material.
Preferably said graded refractive index features comprise a first set of graded refractive index lenses having their optical axes extending along a first direction which is inclined with respect to the plane of the screen and a second set of graded refractive index lenses having their optical axes extending along a second direction which is inclined with respect to the plane of the screen.
Whilst, in some embodiments of the invention, the polymerisable material used is such that polymerisation can be effected or initiated by exposure to light, and in particular to ultraviolet light, it will be appreciated that polymerisable material may be used which is such that polymerisation can be effected by other forms of radiation, for example by electron beams or ion beams. The term xe2x80x9cradiation polymerisablexe2x80x9d as used herein is intended to include such variants as well as materials polymerisable by ultraviolet light.
Preferably, during the exposure of said material, it is exposed at least principally to polymerising radiation from two directions which are equally and oppositely inclined with respect to a plane which is perpendicular to the plane of the sheet of material. However, in some embodiments of the invention, the material is exposed to radiation from a single direction inclined with respect to the plane of the material.
Diffusion or rear projection screens in accordance with some embodiments of the invention can be made to have a pronounced asymmetry. Further, by variation of such factors as the incident angles of the light used for exposure of the photopolymer, it is possible to tailor the degree of asymmetry as well as other parameters, such as xe2x80x9cstrengthxe2x80x9d of diffusion (e.g. fwhm).